The present invention relates to hot melt adhesives, and more specifically to hot melt adhesives having improved hydrophilic properties and which find usefulness in the manufacture of disposable nonwoven articles.
Nonwoven fabric is comprised of an interlocking fiber network, and is employed in the construction of disposable goods. Specific applications of nonwovens have included disposable diapers, sanitary napkins, surgical drapes, hospital pads and adult incontinence products.
In such applications it is generally necessary to adhere nonwoven, tissue, absorbent fluff or the like to another substrate. This second substrate may be another nonwoven fabric, tissue, or a material such as a polyolefin e.g. a polyethylene or polypropylene layer. Typically, a hot melt adhesive has been used to bond such materials together since there is no evaporation step necessary during manufacture, as would be the case for water-based or solvent-based adhesives. Suitable hot melt adhesives must possess the appropriate bond strength to adhere the substrates involved, and must also possess good flexibility, no staining or bleed through, suitable viscosity and open time to function on commercial equipment, acceptable stability under storage conditions, and acceptable thermal stability under normal application conditions.
Many different polymers have been used in hot melt adhesives employed in the construction of disposable nonwoven goods. In this regard typical hot melt adhesives have employed polymers which have included S-I-S (styrene-isoprene-styrene); SBS (styrene-butadiene-styrene); SEBS (styrene-ethylene-butylene-styrene); EVA (ethylene vinyl acetate); and APAO (amorphous poly alpha olefin). While these polymers, when properly blended, provide acceptable adhesion between most substrates employed in typical nonwoven construction such as diapers, and further provide acceptable adhesion under dry conditions, they have had several shortcomings which have detracted from their usefulness.
One of the most noteworthy shortcomings of prior hot melt adhesives concerns the manner in which the adhesive, which is typically very hydrophobic, reacts when exposed to liquids, such as water, urine, or the like. Normally, one would expect the hydrophobic character of hot melt adhesives to be an advantage since such adhesives will provide good dry bonds and will normally maintain an acceptable bond strength when wet. However, manufacturers of disposable nonwoven articles such as diapers have endeavored to produce products which are much thinner in their overall thickness and profile and which incorporate super absorbent materials in place of fluff, which is normally in the core. Thus, it is extremely important in such nonwoven constructions to insure that water, urine or other water-based discharges or solutions are directed toward the absorbent core as quickly as possible, and that any material that might hinder such action be eliminated or at least minimized. As a result, one can now readily understand why the hydrophobicity of typical hot melt adhesives is undesirable since it is a characteristic which inherently hinders fluid transfer into the core of such articles.
Therefore, it has long been known that it would be desirable to have a hot melt adhesive which is useful for bonding to substrates which are typically employed in the construction of nonwoven articles, such as polyethylene, polypropylene, nonwoven, tissue, or fluff, and which further maintains acceptable wet bond strength following exposure for prolonged periods of time to water, urine or similar materials. At the same time, such adhesives should be more hydrophilic to not hinder fluid transfer into the absorbent core of such articles.
It is therefore an object of the present invention to provide an improved hot melt adhesive which is useful for the manufacture of disposable nonwoven articles.
A further object of the present invention is to provide a hot melt adhesive which can be employed as a construction adhesive, and which further will be sufficiently hydrophilic to aid in fluid transfer into the absorbent core of disposable nonwoven articles.
In order to accomplish the above objects, the present invention provides a hot melt adhesive having the following composition (by weight):
about 10-50% of a polymer;
about 40-80% of a tackifying resin;
about 0-40% of a plasticizer;
about 0.1-2% of an antioxidant; and
about 0.1-30% of a surfactant, the components totaling 100% by weight. The surfactant has a hydrophilic-lipophilic balance (HLB) number of less than 15, and is incorporated into the composition in an amount such that the resultant adhesive has a contact angle of 75xc2x0 or less, and preferably less than about 40xc2x0. A low contact angle is desirable so that water, urine or other water-based discharges xe2x80x9cwet outxe2x80x9d rather than xe2x80x9cbead upxe2x80x9d resulting in the fluid being directed away from the adhesive.
It will also decrease the chance of fluid leakage from such articles and help improve the absorption in thinner superabsorbent filled articles by directing the fluid more quickly toward the core. This is accomplished by applying the hydrophilic hot melt adhesive of the present invention to one substrate of a disposable nonwoven absorbent article in a configuration such that fluid is directed toward or xe2x80x9cwickedxe2x80x9d toward a desired absorbent location in one of the substrates. For example, if the disposable article is a diaper, the adhesive might be applied in a rectangular pattern centrally in the crotch region of a nonwoven substrate. This would result in urine being directed toward the crotch region of the absorbent core, rather than the leg cuff region or waist region of the diaper.
The adhesives of the instant invention are especially suited for use in absorbent products such as diapers, training pants, incontinent products, feminine care products, and medical products. With all of these products there is a need to bond the layers or substrates of the article together and hot melts are often used as discussed above. Usually the core area of the article is adhered by spraying a layer of adhesive onto a nonwoven substrate and adhering it to an absorbent core. In many cases, a layer of tissue is placed between the nonwoven and the core, sometimes fully wrapping the core and in other cases simply covering the top layer. Another layer of adhesive may be used to bond the absorbent core fluff to the tissue and further another layer of adhesive may bond the tissue or fluff to the backsheet (which is often polyethylene or a composite laminate). So there is at least one and often a number of layers of sprayed hot melt used in bonding the core into place. If the nonwoven topsheet and core are not in good contact, xe2x80x9ctentingxe2x80x9d of the nonwoven can occur, which can dramatically decrease absorption.
The adhesive further holds the core in place and can help to prevent core cracking. Core cracking can prevent the proper flow of liquids within the core itself and cause leakage. To improve resistance to wet debonding and core cracking, a distinct class of hot melts has been developed having high wet bond strength which represent an improvement over conventional adhesives since they retain the bond to both the tissue and the core when wet.
Unfortunately, the process of placing layers of sprayed hot melt in the area of the core can actually decrease the fluid uptake, since the adhesive itself is quite hydrophobic. Although the new class of high wet bond strength adhesives mentioned above are resistant to debonding under wet conditions, they have also been hydrophobic. The adhesives of the instant invention are very hydrophilic and eliminate this shortcoming, even improving the fluid acquisition of the core over no adhesive (see Examples 2-3). They also can be formulated to be hydrophilic and also have improved the resistance to debonding under wet conditions, like the new class of adhesives mentioned above (see Example 4).
These adhesives show obvious utility in the core area bonding each of the layers in a particular absorbent article together without inhibiting the absorbency of the structure. Absorbent cores can be found in articles such as diapers, incontinent products, feminine care products, medical devices and the like. Often superabsorbents (SAP) as used in absorbent articles to improve the rate and holding power of the cellulosic absorbent material in the core. These SAPs are sometimes bonded into place with hot melts. As can be readily appreciated, these hot melts can slow down the rate of absorbency of the SAP due to their hydrophobic nature. The hydrophilic adhesives of this invention could also be used in this application.
The top layer of absorbent articles needs to allow fluids to readily pass through it. This layer is often a nonwoven or some type of fabric based on polymers such as polyethylene or polypropylene. While these fabrics and appertured films work well as coverstocks, it will be recognized that they are by nature hydrophobic. A bead of liquid placed on these materials will not easily pass through even though the fabric has many open pores. To get around this obvious problem, manufacturers of these materials have had to treat the materials to make them more hydrophilic (often by a surface treatment). In other cases, the manufacturers of absorbent articles have sprayed surfactants or other materials onto the materials during processing. Spraying these materials causes problems in housekeeping of the manufacturing line and the atomized materials can fill the air and be an irritant to operators. These materials also have no use other than to make the topsheet hydrophilic. Also, these surfactants can migrate to other parts of the article and cause the hot melts to debond. It will be noted that the adhesives of this invention could be used both to bond the article together and also to allow use of standard hydrophobic topsheets. The hydrophilic character could be carried to the article to selectively treat areas, allowing greater freedom of absorbent article design.
There are other applications where it is desirable to make a material breathable, that is, to allow moisture (such as sweat) to flow through a laminate. This may be useful both in articles such as diapers and in medical articles. This invention could also be used to improve flow over typical hydrophobic hot melts in such applications.